Many power electronic applications have inductive loads that are driven by power MOSFETs. These MOSFETs are typically in a configuration shown in FIG. 1. When one of the MOSFETs (e.g., s1) is turned off, the inductive load causes the load current to flow through the internal diode of the second MOSFET (i.e., s2). These diodes, known as body diodes, have a minimum forward drop of about 0.6V. The MOSFET channel, however, due to its resistive characteristic, has a drop that is proportional to the current. Thus, in many cases turning the channel ON causes a lower drop across the device, which in turn, reduces the power dissipation.
This method of using the channel to carry the reverse current flow is well known and is called synchronous rectification in applications that create a DC output voltage such as DC—DC converters and synchronous recirculation in applications that drive a motor. An abstract synchronous rectifier is illustrated in FIG. 2. A switch S3 is open to permit diode D1 to block reverse current, and closes when diode D1 conducts a forward current. Accordingly, closure of switch S3 prevents the occurrence of a forward voltage drop of about 0.7 volts across diode D1, and therefore avoids the attendant heat generated by current flowing through diode D1.
In most cases, a controller 16 for switch S3 has to know when the current reverses, so that diode D1 is forward conducting, to control switch S3 to turn on to eliminate the forward voltage drop. The circuit also has to know when the switch current has diminished to zero and then turn switch S3 OFF. If switch S3 is not turned OFF when the current goes to zero, the current may then reverse in many applications (such as Switched Reluctance motors and trapezoidally wound and driven Brushless DC motors) and this can be harmful to the application. In many applications, due to cost or technical reasons, it is difficult for the controller to know if the current has gone to zero to turn the switch S3 OFF.